The present invention relates to a method of tuning and compensating the signal strength indicator of a radio telephone having a logic section controlling the radio telephone's operation.
In radio telephones, a received signal strength indicator (RSSI) is used to measure the strength of a signal received by the telephone's antenna interface. This information is needed to select the appropriate channel of the radio telephone, i.e., the channel corresponding to the frequency of the strongest signal received. During operation, the measurement is performed by measuring a DC voltage level which is proportional to the strength of the intermediate frequency signal produced in the intermediate frequency section of the receiver. In the prior art, this voltage is generally measured using an analog-to-digital (A/D)-converter or comparators, which can identify, e.g., three signals with different strengths.
This type of prior art measurement suffers from two factors which produce a considerable amount of inaccuracy. Temperature variations cause the biggest inaccuracy by causing changes in the amplification in the RF and IF sections of the receiver. The second factor is the unevenness of the RF filter frequency response, which produces different measurement results at different frequencies.
In the prior art, compensation for temperature dependent changes is achieved by using analog technology, e.g., temperature dependent NTC resistors. However, the changes of individual radio telephone units can be somewhat different at various temperatures, and thus the compensating circuitry designed for a mass produced radio telephone will not operate identically for every unit, and the compensating circuitry could function inaccurately, due to component tolerances. Naturally, both of these results are unsatisfactory.
Previously, the frequency response errors of filters have not been corrected in any way, but rather, exceptionally stringent requirements have been imposed on the frequency response in the filter design, thus greatly increasing the size and price of the filters.
Generally, the radio telephone casing has to be opened to tune it in order to reach the various components needing adjustment.